1. Field of the Invention
The present invention relates to a liquid crystal driving device and method, and a liquid crystal display device for driving an active matrix type liquid crystal display panel.
2. Description of Related Art
Active matrix type liquid crystal display panels such as a TFT liquid crystal display panel have switching elements such as a TFT, liquid crystal capacitors CLC, and an auxiliary capacitor CS at intersections between gate lines (scan lines) and data lines (signal lines) arranged in matrix. The following description focuses on the TFT liquid crystal display panel by way of example. FIG. 13 shows an equivalent circuit of the TFT liquid crystal display panel.
A TFT 110 has a gate electrode G connected to a gate line 111, a source electrode S connected to a data line 112, and a drain electrode D connected to a pixel electrode of the liquid crystal capacitors CLC and the auxiliary capacitor CS. The liquid crystal capacitors CLC is a capacitor of a liquid crystal defined between the pixel electrode 113 and a common electrode 114. The auxiliary capacitor CS is used for keeping a predetermined level of voltage applied to a liquid crystal even after the voltage application to a gate was stopped. FIG. 13 shows an example where the auxiliary capacitor CS is provided between the pixel electrode 113 and the common electrode 114. However, one end of the capacitor CS may be connected with an adjacent gate line, not the common electrode.
FIG. 14 is a waveform chart of a voltage applied to a liquid crystal. FIG. 14 shows how a liquid crystal application voltage VLC per liquid crystal pixel changes its level from one frame to another in the case of inverting a polarity of the liquid crystal application voltage VLC every frame period (frame-inversion driving). Here, a gate voltage VG is a voltage applied to the gate electrode G of the TFT 110. A source voltage VS is a voltage applied to the source electrode S. A common electrode voltage (common voltage) Vcom is a voltage applied to the common electrode 114. Further, the voltage VLC is a voltage applied to the liquid crystal capacitor CLC, which is equivalent to a potential difference between the pixel electrode 113 and the common electrode 114 (hereinafter, referred to as “liquid crystal application voltage”). If a DC voltage is continuously applied to the liquid crystal, a liquid crystal element could burn out and deteriorate. Hence, in driving a liquid crystal display panel, the polarity of the source voltage VS is periodically inverted to invert the polarity of the liquid crystal application voltage VLC at regular intervals. Such Polarity-inversion driving gives the amplitude of the source voltage VS that is twice the amplitude obtained without inverting the polarity. In some cases, as shown in FIG. 14, common-inversion driving is executed to invert the polarity of the common voltage Vcom in sync with a timing of inverting the polarity of the source voltage VS so as to obtain the amplitude of the source voltage VS equivalent to the amplitude obtained without inverting the polarity.
The liquid crystal application voltage VLC varies depending on a difference between the source voltage VS and the common voltage Vcom at the time of gate-off (when a potential of the gate voltage VG is switched to a “Low” level) but is unequal to the difference, to be exact. This is because, owing to the presence of a gate-drain parasitic capacitance CGD, charges accumulated in the liquid crystal capacitors CLC are stored in a gate-drain parasitic capacitance CGD, with the result that a level of the liquid crystal application voltage VLC is changed. To be specific, as shown in FIG. 14, a voltage shift ΔV1, or ΔV2, occurs with respect to the liquid crystal application voltage VLC. Here, the voltage shift ΔV is represented by Expression 1 below.ΔV=ΔVG(CGD/(CGD+CLC+CS))  (Expression 1)where ΔVG represents a variation of the gate voltage VG between in the gate-on state and in the gate-off state. As apparent from Expression 1 above, the voltage shift ΔV varies depending on a capacitance value of the liquid crystal capacitor CLC. On the other hand, the liquid crystal application voltage VLC varies depending on a voltage value of the source voltage VS. Accordingly, the voltage shift ΔV varies depending on the source voltage VS.
Considering this example with reference to FIG. 14, an image is displayed with the same gray scale during first and second frames, so the source voltage VS is constant albeit its polarity is inverted, and an amount of the shift ΔV is constant (ΔV1). However, during third and fourth frames, a gray scale of a display image is changed by changing a value of the source voltage VS from that in the second frame. As a result, the amount of the voltage shift is changed from ΔV1 to ΔV2.
As shown in the waveform chart of FIG. 14, a difference is caused between a voltage amplitude Vp1 with a positive polarity (first frame) and a voltage amplitude Vn1 with a negative polarity (second frame) even if an image is displayed with the same gray scale. Further, there is a difference between voltage amplitude Vp2 in the third frame and a voltage amplitude Vn2 in the fourth frame. Such a difference between the negative polarity and the positive polarity of the liquid crystal application voltage VLC causes not only flickering of a display image but burning due to the application of the DC voltage to the liquid crystal. Incidentally, such a difference between the negative polarity and the positive polarity of the liquid crystal application voltage VLC due to the voltage shift ΔV is also caused in the case where the auxiliary capacitor CS is defined between the pixel electrode 113 and an adjacent gate line.
To that end, there has been proposed a technique of eliminating the difference between the negative polarity and the positive polarity of the liquid crystal application voltage VLC, in other words, removing DC components of the liquid crystal application voltage VLC by adjusting the common voltage Vcom. For example, Japanese Unexamined Patent Application Publication No. 2000-267618 discloses a liquid crystal display device that adjusts a DC voltage level of the common voltage Vcom based on a video signal voltage for displaying an image on a liquid crystal display panel to reduce a voltage difference between the negative polarity and the positive polarity of the liquid crystal application voltage VLC. A technique of adjusting the source voltage VS to remove the DC components of the liquid crystal application voltage VLC has been also proposed (see Japanese Unexamined Patent Application Publication No. 2003-114659).
As mentioned above, there has been known the liquid crystal display device that adjusts a value of the common voltage Vcom to remove the DC components of the voltage liquid crystal application voltage VLC to eliminate the difference between the negative polarity and the positive polarity of the voltage VLC. However, the known liquid crystal display device has a problem that a timing of adjusting the value of the common voltage Vcom for removing the DC components of the liquid crystal application voltage VLC cannot be controlled.
For example, Japanese Unexamined Patent Application Publication No. 2000-267618 discloses a technique of amplifying an average picture level (APL) signal corresponding to an average voltage in one frame period of a image display signal, and overlapping the amplified APL signal on an output of a common electrode driving amplifier for driving a common electrode to adjust a center voltage of the common voltage Vcom. However, in the structure disclosed in Japanese Unexamined Patent Application Publication No. 2000-267618, a horizontal or vertical control signal generated by an LCD controller is not referenced upon adjusting the common voltage Vcom.
In the structure disclosed in Japanese Unexamined Patent Application Publication No. 2000-267618, the timing corresponding to a vertical clock signal V and horizontal clock signal H extracted from the input image display signal is different from the driving timing of a signal driver and scan driver at the actual display time of the liquid crystal display panel. This is because the signal driver and scan driver drive a data line or gate line through a processing for moving input image data to an output position, and a processing for converting the input image data into a signal voltage applied to the liquid crystal. Thus, in the structure disclosed in Japanese Unexamined Patent Application Publication No. 2000-267618 where the horizontal or vertical control signal generated by the LCD controller is not referenced upon adjusting the common voltage Vcom, the timing of adjusting the common voltage Vcom cannot be decided in consideration of the timing of driving the data line or gate line. Hence, it is difficult for the structure disclosed in Japanese Unexamined Patent Application Publication No. 2000-267618 to adjust the common voltage Vcom under control exclusively during a blanking period in which neither data lines nor gate lines in a display area of the liquid crystal display panel are driven. Therefore, there is a possibility that the common voltage Vcom changes in the middle of displaying an image on the liquid crystal display panel.
If the common voltage Vcom is changed during a period (scanning period) in which an image is being displayed on the liquid crystal display panel, without controlling the timing of adjusting the common voltage Vcom, flickering occurs in a display image due to an abrupt luminance change, leading to deterioration of an image quality. Therefore, it is desirable to control the timing of adjusting the common voltage Vcom such that the adjustment is carried out during the blanking period.